Thumbnail submission not working

Hi Anh !
I've created a nice frontend form that works pretty well excepted that the thumnail is not uploading.
If I add a picture from the WP dashboard, I see it in my frontend form, but if I choose a picture from the frontend form, it's not uploading, and return no error.
As you mentioned in the documentation, I've checked that the user has the capacity 'upload_files' (echo current_user_can( 'upload_files' ) returns 1).

Any idea ? Thanks !
Cedric.